Despite law enforcement's presence, a brawl between players broke out and involved at least one fan.

Cowboys cornerback Morris Claiborne laid a lick on Raiders tight end Mychal Rivera and players from both teams started to push and shove near a sideline and up against a fence where Raiders fans had gathered.

Dallas cornerback B.W. Webb told USA TODAY someone swung a helmet at the back of his head. When he spun around to see who was trying to attach him, Webb said it was a fan who had swung a helmet at him.

"That's definitely a dangerous situation," Webb said. "I don't know why fans would want to jump on this side of the fence. I don't think they'd like it too much on this side."

Oxnard, in Ventura County, has an ordinance in place preventing people from wearing Cowboys or Raiders jerseys in specific areas. Two of the county’s most feared and violent gangs are known for wearing Cowboys and Raiders clothing.

Claiborne said he was on the bottom of the pile and was just hoping no one would rip his helmet off.

Another brawl brought an end to the joint practice. Cowboys fullback J.C. Copeland and Raiders linebacker Justin Cole were the main participants.

"That passion is what I had hoped to get out of getting these teams together," Cowboys owner Jerry Jones told the Dallas Morning News. "This was great, the enthusiasm, the atmosphere. It’s hard to believe this was a practice."